"lena has a spiral fracture of her tibia just above where her ski boot ends. what is a spiral fracture, and why might this be a common injury to the tibia in skiing

Spiral fracture is a torsion fracture

Explanation:

A spiral fracture is a type of bone fracture that happens when a rotating force (torque) is exerted along the axis of a bone. This fracture occurs mostly in bodily motions and  is caused by a twisting force while one leg is fixed and the other is rotated along the axis.

The tibia is a larger and  stronger bone, situated at the anterior part of the two bones in the leg below the knee region. The tibia provide support to body weight.

During Skiing (a sport recreational activities by the aid of using skis to glide on snow), a rotating force is (torque) is exerted along the axis of this tibia  bone which is common and capable of causing injury to the tibia when skiing.

the answer is; comparing the frequency of collisions with and without the device

this makes the anti-collision system an independent variable and the collisions a dependent variable. if there are fewer collisions with an anti-collision system that without, then it can be stated that the system is a success. essentially, collisions under an anti-collision system are compared with the frequency of collisions by cars with the human drivers.